The easiest method for forming a semiconductor epitaxial layer is generally to form a single crystal substrate made of the same material as that of a semiconductor to be grown, and to cause a vapor-phase growth of a semiconductor crystal on the resultant substrate, with actual success in various materials. However, because the single crystal substrate is not only technically difficult to obtain but also high in cost, a semiconductor crystal different from the substrate should often be caused to grow on the substrate. In such a case, known combinations of the substrates and the semiconductor crystals are, for instance, GaAs on a silicon substrate, a nitride semiconductor on a sapphire or silicon carbide substrate, II-VI group semiconductors on a GaAs substrate, etc.
However, when a semiconductor different from a substrate is caused to grow on the substrate, dislocations are likely introduced in a high density into a grown semiconductor epitaxial layer, because of the mismatch of various characteristics such as lattice, a thermal expansion coefficient, surface energy, etc. Because the dislocations in the semiconductors may constitute non-radiative recombination centers, scattering centers, etc. in semiconductor devices such as optical devices, electronic devices, etc., devices using high-dislocation-density semiconductors are extremely poor in characteristics and stability.
Because bulk crystal growth is also difficult in nitride semiconductors typically including gallium nitride (GaN), aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN), etc., single crystal substrates with practically acceptable sizes have not been obtained yet. Accordingly, methods for causing nitride semiconductor crystals to grow epitaxially on substrates such as sapphire, silicon carbide, etc. are generally used. Still, dislocations remain serious problems as described above.
Recently proposed (JP 8-8217 B, etc.) is the reduction of a dislocation density in a nitride semiconductor layer by “a two-stage growing method” using a metal-organic vapor-phase epitaxy method (MOVPE method). FIG. 4 is a graph showing temperature variation at the time of crystal growth in a conventional two-stage growing method. In this method, after a hydrogen gas is blown at as high a temperature as 1000 C or higher onto a substrate surface of sapphire, etc. to remove an oxide film from the surface (hydrogen cleaning), (i) a low-temperature buffer layer made of GaN, AlN, etc. is caused to grow at 500-600 C on the substrate, (ii) the temperature is elevated to about 1000 C (annealing), and (iii) a nitride semiconductor layer made of GaN, etc. is caused to grow at about 1000 C. In the step (i), the crystal-growing temperature is lower than the melting point of GaN, AlN, etc., resulting in the formation of a polycrystalline, low-temperature buffer layer. In the subsequent step (ii), the low-temperature buffer layer is partially turned to a single crystal by elevating the temperature to about 1000 C. In the step (iii), an epitaxial layer made of GaN, etc. is formed with this single crystal as nuclei. According to this method, the dislocation density of GaN on a sapphire substrate can be reduced from 1010-1011 cm−2 to about 109 cm−2.
However, the reduction of a dislocation density by the two-stage growing method is still insufficient, and the dislocation density often cannot be reduced sufficiently depending on the types and structures of a crystal-growing apparatus. Conditions suitable for the reduction of dislocations may be achieved in some particular crystal-growing apparatuses. Even in such a case, the apparatuses may be subjected to variations in conditions, etc. for a long period of operation, affecting crystal growth conditions. Therefore, it is difficult to stably grow nitride semiconductor layers with small dislocation densities.